China is situated on the eastern third of the Eurasian landmass, between Russia, mainland Southeast Asia, the Indian subcontinent and Central Asia. Its more than 9,000 mile-long coastline abuts the Yellow, East and South China Seas. China is a country of deep geographic divisions. Most fundamental is the split between its fertile eastern lowlands and the arid, sparsely populated highlands that enclose the lowlands like a shell. More than a billion people live in the ethnic Han Chinese core, making it one of the most densely populated places on Earth. Traditionally, threats to China's Han core originated in the borderlands. To guard against overland invasion, successive Chinese rulers have sought to push the Core's borders outward —integrating these highlands as strategic "buffer" zones. These zones form a shield, protecting and containing the core. To be secure, China must control the buffer regions. But maintaining control of the regions, in turn, requires a strong and united core. And that means overcoming immense internal divisions — not only between northern and southern regions orbiting the Yellow and Yangtze rivers, but also between smaller regional units, each with their own geography, history, dialect and interests. Chinese history is defined by cycles of unity and fragmentation, from periods when a strong Han core captures and holds the surrounding buffers to those when a weak core breaks into its constituent parts, loses internal coherence and cedes control of the borderlands. This pattern, rooted in China's geography, has played out with remarkable consistency. By comparison, China's maritime interests have remained mostly limited to coastal waters. Today, however, growing international trade and rising Chinese reliance on overseas resources threaten to alter the pattern, adding a new maritime dimension to the struggle for buffer space — and potentially upending the long-standing dynamics of China's geographic challenge.